Prior permanent magnet direct current motors are explained in detail in a book entitled: "Brushless Permanent Magnet Motor Design" by Duane C. Hanselman, published in 1994 by McGraw Hill, Inc., New York. All commercial brushless, permanent magnet direct current motors known to date operate from external variable voltage, variable frequency sources. The air gap fields produced by permanent magnets cannot be externally controlled such that the electromotive forces (emf) or back-electromotive forces (back-emf) of stator windings are strictly functions of speed. The term emf is customarily used for a generator while back-emf is used for a motor. Both terms refer to the same effect caused by the rate of change of flux linkage with a winding which is proportional to rotational speed.
The amplitude of the air gap field in a PM motor is practically constant under normal operating conditions. As the rotor speed increases, so does the back-emf of the motor windings. In the following relationships E.sub.emf or E.sub.bemf =Blv where B is air gap field density, l is the length of winding wire coupling the field B and v is the tangential speed of the rotor pole face at the air gap. EQU E.sub.dc -iR.sub.w -L.sub.w di/dt-E.sub.bemf =0
or EQU E.sub.dc -iR.sub.w -L.sub.w di/dt-Blv=0
where E.sub.dc is supply voltage, E.sub.bemf and Blv are back-emf voltage, i is winding current and R.sub.w and L.sub.w are winding resistance and inductance. Disregarding the inductance term for steady state conditions and solving for winding current: EQU i=(E.sub.dc -Blv)/R.sub.w, and considering that motor torque T=Blir where B is air gap
field density, l is the length of winding wire coupling the field B, i is winding current and r is the air gap radius. As the rotor speed increases, so does the back-emf voltage resulting in reduction of winding current if the supply voltage is constant. Thus, with constant supply voltage, the torque is reduced as the winding current is reduced with increasing rotor speed. To overcome this limitation in torque, power converters driving prior art PMDC motors must boost the output voltage to the winding. This increases the complexity and robustness of the motor controller and the degree of voltage stress and heat generation concentrated in the controller power semiconductors. The switching components must then have high voltage ratings as well as high current ratings.
In a PM generator the amplitude of the air gap field is also practically constant. For a direct current generator driving a resistive load RL the mathematical relatioship is: EQU E.sub.emf -iR.sub.w -L.sub.w di/dt-iR.sub.L =0
or EQU Blv-iR.sub.w -L.sub.w di/dt-iR.sub.L =0
where its air gap voltage E.sub.emf =Blv, i is winding current and R.sub.w and L.sub.w are winding resistance and inductance. Disregarding the inductance term for steady state conditions, Blv-i R.sub.w =i R.sub.w =iR.sub.L and the expression on the left side of the equal sign is the generator output voltage to the load. Therefore, at constant rotational speed the output voltage of a PM generator is constant, its emf being a function of speed.
It is practically impossible to have perfectly uniform and balanced air gap field intensities and distributions produced by permanent magnets. In prior art PMDC machines this condition, combined with the high rates of change of magnetic coupling caused by switching distributed phase windings cause several undesirable parasitic effects. The most objectionable of these effects is torque pulsations or torque ripple. With trapezoidal or asymmetrical air gap field distributions and phase current waves distributed over the pole width, prior art PMDC machines have large components of space harmonics. Those harmonics induce circulating currents in the rotor and high core losses in the stator. Therefore, undesirable losses composed of hysterisis and eddy currents take place in the rotor and the stator core. To overcome those parasitic effects, technologies involving pulse width modulation, multi-level power converters or power conditioners are used to make the controller outputs closer to sine waves. This limits the utilization of magnetic core circuits in those motors to approximately 60 percent. The following references address the problems cited above for prior art motors driven by external variable voltage, variable frequency power converters:
IEEE Conference Paper, Titled: Performance Analysis of Permanent Magnet Brushless DC Motor, Authors: Miraoui, A.; Lin DeFang; Kauffman, J. M.
IEEE Transactions on Industrial Electronics, VOL 43, No. 2, April 1996, Titled: Identification and Compensation of Torque Ripple in High-Precision Magnet Motor Drives, Authors: Holtz, Joachim and Springob, Lothar.
1994 Institution of Electrical Engineers, Title: Adverse Electrical Phenomena in Rail Traction Using Alternating Current Motors, Authors: Minalescu, D. and Pantelimon, M.